High entropy alloy structure and a method of preparing the same

ABSTRACT

A method for preparing a high entropy alloy (HEA) structure includes the steps of: preparing an alloy by arc melting raw materials comprising five or more elements; drop casting the melted alloy into a cooled mold to form a bulk alloy; applying an external force against the bulk alloy to reshape the bulk alloy; and heat-treating the reshaped bulk alloy, wherein the bulk alloy is reshaped and/or heat-treated for manipulating the distribution of the microstructure therein. The present invention also relates to a high entropy alloy structure prepared by the method.

TECHNICAL FIELD

The present invention relates to a high entropy alloy structure and amethod of preparing the high entropy alloy structure, specifically,although not exclusively, to a high entropy alloy with heterogeneouseutectic microstructures and a method of preparing a high entropy alloywith heterogeneous eutectic microstructures.

BACKGROUND

With respect to the human history, human civilization has striven todevelop, discover and invent new materials for more than thousands ofyears. Since the Bronze Age, alloys have traditionally been developedaccording to a “base element” paradigm. That is, choosing one or rarelytwo principle elements such as iron in steels or nickel in superalloysfor its properties, and a minor alloying approach to obtain the alloys.The alloys obtained usually have either superior strength or superiorductility. An alloy with high strength may be used in constructingautomotive parts such as crossmembers, shock towers, crush cans, etc.whereas an alloy with high ductility may be used in manufacturing toolswith various shapes.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is a block diagram showing the process flow of a method forpreparing a high entropy alloy structure in accordance with oneembodiment of the present invention;

FIG. 2A is a scanning electron microscopy image of an as-casthypoeutectic high entropy alloy Co₃₀Ni₃₀Cr₁₃Fe₁₃Al₁₄ as prepared inaccordance with one embodiment;

FIG. 2B is a scanning electron microscopy image of an as-casthypoeutectic high entropy alloy Co₃₀Ni₃₀Cr₁₂Fe₁₂Al₁₆ as prepared inaccordance with one embodiment;

FIG. 2C is a scanning electron microscopy image of an as-cast fullyeutectic high entropy alloy Co₃₀Ni₃₀Cr₁₁Fe₁₁Al₁₈ as prepared inaccordance with one embodiment;

FIG. 2D is a scanning electron microscopy image of an as-casthypereutectic high entropy alloy Co₃₀Ni₃₀Cr₁₀Fe₁₀Al₂₀ as prepared inaccordance with one embodiment;

FIG. 3 is an X-ray diffraction diagram showing the X-ray diffractionpatterns of the fully eutectic high entropy alloys Co₃₀Ni₃₀Cr₁₁Fe₁₁Al₁₈as prepared in accordance with one embodiment; and

FIG. 4 is a plot of engineering stress against engineering strainshowing the tensile engineering stress-strain curves of the fullyeutectic high entropy alloys Co₃₀Ni₃₀Cr₁₁Fe₁₁Al₁₈ as prepared inaccordance with one embodiment.

SUMMARY OF THE INVENTION

In accordance with the first aspect of the present invention, there isprovided a method of preparing a high entropy alloy structure comprisingthe steps of: preparing an alloy by arc melting raw materials comprisingfive or more elements; drop casting the melted alloy into a cooled moldto form a bulk alloy; applying an external force against the bulk alloyto reshape the bulk alloy; and heat-treating the reshaped bulk alloy;wherein the bulk alloy is reshaped and/or heat-treated for manipulatingthe distribution of the microstructure therein;

In an embodiment of the first aspect, step C includes step C1 of rollingthe bulk alloy along a first direction to reduce the thickness of thebulk alloy;

In an embodiment of the first aspect, step C1 of rolling is carried outalong a longitudinal direction of the bulk alloy;

In an embodiment of the first aspect, the thickness of the rolled bulkalloy is reduced by 70%;

In an embodiment of the first aspect, formed bulk alloy includes ahomogenous structure within which the microstructures are uniformlydispersed;

In an embodiment of the first aspect, heat-treated bulk alloy includes aheterogeneous structure within which the microstructures arenon-uniformly dispersed;

In an embodiment of the first aspect, the crystals in the microstructureare deformed during the heat treatment in step D to form a plurality oftwins;

In an embodiment of the first aspect, step D includes step D1 of heatingthe bulk alloy to facilitate the movement of the microstructures;

In an embodiment of the first aspect, step D includes step D2, afterstep D1, of water quenching the heat-treated alloy;

In an embodiment of the first aspect, each of the elements is providedin an atomic percentage of 10% to 30%;

In an embodiment of the first aspect, the elements are Cobalt, Nickel,Chromium, Iron and Aluminum;

In an embodiment of the first aspect, Cobalt, Nickel, Chromium, Iron andAluminum are provided in an atomic ratio of 30:30:20-0.5x:20-0.5x:x,with X being an integer of 14 to 20;

In an embodiment of the first aspect, the raw materials have a highpurity of >99.90%;

In an embodiment of the first aspect, step A includes step A1 offlipping and re-melting the raw materials in a repetitive manner;

In an embodiment of the first aspect, the mold is made of copper;

In an embodiment of the first aspect, the alloy is arc melted within aTi-gettered argon atmosphere with a pressure below 8×10⁻⁴ Pa;

In an embodiment of the first aspect, the rolled bulk alloy is annealedat a temperature of at least 800° C. for 6 hours;

In accordance with the second aspect of the invention, there is provideda high entropy alloy structure prepared by the method in accordance withthe first aspect;

In an embodiment of the second aspect, the alloy structure includeslamellar structures;

In an embodiment of the second aspect, the size of the lamellarstructures is provided in submicron range;

In an embodiment of the second aspect, the alloy structure possesseshardness of 330 to 404 HV;

In an embodiment of the second aspect, the yield stress of the alloystructure is around 850 to 1000 MPa;

In an embodiment of the second aspect, the Young's modulus of the alloystructure is around 230 GPa;

In an embodiment of the second aspect, the alloy structure is thermalstable up to a predetermined temperature of 900° C.;

In an embodiment of the second aspect, the structure is a dual phaseeutectic structure; and

In an embodiment of the second aspect, the dual phase includes orderedface center cubic (FCC) phase and body center cubic (BCC) phase.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

High Entropy Alloys (HEAs) are a new kind of alloy typically composed offive or more elements with near equi-atomic ratio and noprincipal/dominant element. These alloys, however, usually possessrelatively a single phase structure, which may lead to a failure incombining different mechanical properties such as strength andductility.

Without wishing to be bound by theories, the inventors have, throughtheir own research, trials, and experiments, devised a new alloymaterial, eutectic high entropy alloys (EHEAs) and a method of preparingthe same. The EHEAs may contain multiphase structures with submicronranges. Comparing with conventional eutectic alloys, the EHEAs having aparticular structural orientation in each phase may result in asynergistic effect of multicomponents such that optimal mechanical andfunctional properties may be achieved.

With reference to FIG. 1, there is provided a block diagram showing theprocess flow of a method for preparing a high entropy alloy (HEA)structure. The method comprises the steps of: preparing an alloy by arcmelting raw materials comprising five or more elements; drop casting themelted alloy into a cooled mold to form a bulk alloy; applying anexternal force against the bulk alloy to reshape the bulk alloy; andheat-treating the reshaped bulk alloy. The bulk alloy is reshaped and/orheat-treated for manipulating the distribution of the microstructuretherein

As shown, in step 102, an alloy is prepared by arc melting raw materialscomprising five or more elements. The raw materials may be independentlyselected from the elements of groups 4-13 in period 3-6 in the periodictable or the elements of lanthanide series in the periodic table,particularly from the elements of groups 4-13 in period 3-6, preferablyfrom the elements of groups 4-13 in period 3-4.

Most preferably, the raw materials are Cobalt, Nickel, Chromium, Ironand Aluminium. Each of the elements may be provided in an atomicpercentage of 10% to 30%. Preferably, the raw materials are providedaccording to an atomic ratio of 30:30:20-0.5x:20-0.5x:x with x being aninteger of 14 to 20. Specifically, the raw materials, Cobalt, Nickel,Chromium, Iron and Aluminum are provided with an atomic percentage of30%, 30%, 10-13%, 10-13%, and 14-20%. The raw materials may be of a highpurity such as >90%, particularly >95%, preferably >99%, most preferably>99.90%.

The aforementioned raw materials may be melted in an arc furnace underan inert atmosphere. Preferably, the arc furnace is pump-filled withTi-gettered argon gas, for example, 5 times such that the pressureinside the furnace is less than 8×10⁻⁴ Pa.

During the arc melting in step 102, the raw materials may be flipped andremelted in a repetitive manner in step 104 so as to ensure chemicalhomogeneity. In other words, to ensure each of the raw materialcomponents are uniformly distributed. Preferably, the raw materials areflipped and re-melted for at least five times.

Once the raw materials are completely arc melted, the resultantmaterial, that is the melted alloy, may be drop casted into a cooledmold to form a semi-finished product in step 106. Preferably, the meltedalloy may be drop casted into a copper mold cooled with water so as toobtain a bulk alloy. The bulk alloy obtained in step 106 may include ahomogeneous structure within which the microstructures are uniformlydispersed.

After obtaining the bulk alloy, the bulk alloy may be reshaped and/orheat-treated in steps 108 and 109 so as to manipulate the distributionof the microstructures. In step 108, the bulk alloy may be reshaped byapplying an external force against the bulk alloy. In this step, arolling process may be carried out to reshape the bulk alloy. The term“rolling” refers to a process of which a bulk metal is passed throughone or more pairs of rolls to reduce the thickness of the metal and tomake the thickness uniform. In particular, the rolling process may beperformed at a temperature above or below the recrystallizationtemperature of the bulk metal. In other words, the bulk metal may bereshaped by a hot rolling process or a cold process. Preferably, thebulk alloy obtained in step 106 is subjected to a cold rolling processalong a longitudinal direction of the alloy. As such, the thickness ofthe alloy is substantially reduced by, for example, 70%. That is, arolled alloy with a thickness of which is reduced by 70% after step 108.

The rolled alloy may be subjected to a specific heat treatment 109 so asto further manipulate the distribution of the microstructures therein.The heat treatment involves steps 110 and 112. In step 110, the bulkalloy is annealed to facilitate the movement of the microstructures. Tocarry out annealing process, the rolled alloy may be heated to at least800° C., in particular to 800° C. or 900° C. for 6 hours in the furnace.In this way, the crystals in the microstructures may be deformed to forma plurality of twins.

The annealed alloy is then taken out from the furnace and directlyquenched with water so as to obtain a bulk alloy with eutecticmicrostructures therein in step 112. The annealed bulk alloy may includea heterogeneous structure within which the microstructures arenon-uniformly dispersed. As such, a stable microstructure may beadopted, which may result in enhanced mechanical and thermal propertiesfor the high entropy alloy.

As mentioned above, the bulk alloy formed in step 106 may have ahomogeneous microstructure within which the microstructures areuniformly dispersed. This may be done by systematically varying theAluminium content (as well as Chromium, Iron) of the alloy. Suchvariation may also lead to different morphologies to the HEAs prepared.It is aware by the skilled person in the art that the morphologies ofthe prepared HEAs may be characterized by methods such as scanningelectron microscopy (SEM).

With reference to FIGS. 2A to 2D, there are provided the SEM images ofHEAs prepared by the method as described above. In this example, theHEAs are as-cast alloy obtained in step 106 without undergoing thereshaping process 108 and heat treatment 109. The HEAs are differentfrom each other by their aluminium contents. Preferably, the HEAs 202,204, 206 and 208 possess an aluminium content of 14%, 16%, 18%, and 20%by atomic percentage respectively.

As shown, the morphologies of the HEAs vary as the aluminium contentincreases. All the HEA surfaces were occupied with submicron sizelamellar structures in different extent. With the lowest aluminiumcontent, the surface of HEA 202 was occupied by a few lamellarstructures. There are also some network-like structures connecting thelamellar structure spread through the surface of HEA 202. When thealuminium content increases to 16% by atomic percentage, as shown inFIG. 2B, the network-like structures no longer exists on the surface ofHEA 204. Rather, the surface was occupied by lamellar structuresarranged regularly, i.e. the lamellar structures are spaced apart with apredetermined distance. The failure in occupying the whole surface ofHEAs 202 and 204 by the lamellar structures may indicate that the HEAsare under a hypoeutectic state.

With the aluminium content increased up to 18%, the surface of HEA 206was fully occupied by the lamellar structures. As shown in FIG. 2C, theorientation of the lamellar structures does not follow a particulardirection as compared with those in FIG. 2B. In other words, thelamellar structures of HEA 206 are oriented in all directions. Thischaracteristic may indicate that the HEA 206 is under a fully eutecticstate. Nevertheless, any further increase in the aluminium content, forexample, to 20% by atomic ratio may lead to a negative effect on theformation of lamellar structures on the HEA surface. As shown in FIG.2D, although the surface of HEA 208 was still mostly occupied bylamellar structures, the structures were more loosely packed as comparedwith those in FIG. 2C. In addition, there were some porous areas locatedwithin the lamellar structure network. This feature may be an indicatorthat the HEA 208 is under a hypereutectic state. Advantageously, thelamellar structure of the HEAs 202, 204, 206 and 208 is notsubstantially affected by the reshaping process 108 or the heattreatment 109.

It is believed that due to the high entropy effect at equal ornear-equal atomic ratios, the multicomponents in HEA may tend to formsingle phase structures, which may render the HEA lack of desireproperties.

Without wishing being bound by the theories, the inventors devised thatthe HEA prepared by the aforementioned method possesses multiphasesparticularly dual phases. With reference to FIG. 3, there is provided anX-ray diffraction diagram showing the X-ray diffraction pattern of theHEA structures of the aforementioned embodiments. As shown, the as-castHEA 206 possesses a dual phase structure, namely ordered face centrecubic (FCC) and body centre cubic (BCC) phases. Importantly, even afterthe HEA 206 subjected to the reshaping process 108 and heat treatment109, the structure phases of the resultant HEAs 206A and 206B remainedunchanged.

Advantageously, by having two or even more phases as well as undergoingreshaping and heat-treatment, the HEAs of the present invention may havean excellent mechanical strength such as high strength, hardness, andductility, and thermal stability.

In one embodiment, the hardness of the HEAs may be provided in a rangeof 330 to 404HV. In other words, the hardness of the HEAs may beprovided as high as 404HV. It is aware by the skilled person in the artthat the hardness measurement may be carried out with a microhardnesstester. In other embodiment, the HEAs may be thermal stable up to apredetermined temperature of 900° C. That is, the microstructure of HEAsis stable up to 900° C.

With reference finally to FIG. 4, there is provided a plot ofengineering stress against engineering strain showing the tensileengineering stress-strain curves of the HEAs as prepared by theaforementioned method. It is appreciated that upon a force is applied toa material, the material may undergo different deformation modes (i.e.change in shapes and/or size in different manner). The material mayfirst undergo a reversible deformation, namely elastic deformation inresponse to the applied force. During this process, the original shapeand size of the material may be temporarily changed when a force isapplied and may be restored when the applied force is removed. Thisreversible deformation may continue upon the applied force increasesuntil a threshold is reached, namely yield stress.

Beyond such a yield point, the original shape and size of the materialmay no longer be restored even the applied force is removed. In otherwords, the deformation becomes irreversible and in turn the materialpermanently stays at a particular shape and/or size. The thus-processrefers as plastic deformation. Preferably, a material with high strengthmay have a high yield stress and/or Young's modulus whereas a materialwith high ductility may have a high fracture point (i.e. the engineeringstrain at which the material becomes fracture).

Referring to FIG. 4, the as-cast HEA 206 and HEAs 206A and 206B reshapedand annealed at 800° C. or 900° C. respectively displayed an elasticdeformation behaviour upon external force is applied. Each of the HEAshas a yield stress of around 850 to 1000 MPa and a Young's modulus of230 GPa. In particular, the yield stresses of the reshaped and annealedHEAs 206A and 206B were determined to be higher than that of the as-castHEA 206. Beyond the yield stress, the HEAs underwent plastic deformationand eventually fractured at around 15 to 19% of the engineering strain.Similarly, it is determined that the fracture point of the reshaped andannealed HEAs 206A and 206B were higher than that of the as-cast HEA206. All these results suggest that the reshaping process and heattreatment may contribute to the relatively higher strength and ductilityof the HEAs 206A and 206B, as well as the formability of the HEAs 206Aand 206B i.e. the plastic deformation capacity without being damagedsuch as tearing or fracture.

In one embodiment, the inventors have, through their own research,trials, and experiments, devised that the lamellar structure of theaforementioned reshaped and annealed HEAs may become heterogeneous. Inaddition, the ordered FCC phase may be transformed into FCC structures.During the deformation process, a high density of deformation twinningwas formed as a result of a low stacking energy of the FCC phase. Assuch, a higher strength and ductility may be obtained in view of thesynergistically effect of the heterogeneous structure and the occurrenceof deformation twinning.

The present invention is advantageous in that by subjecting the HEAs toa reshaping process and a heat treatment, the microstructures thereinmay be manipulated which in turn providing an excellent strength andductility, good thermal stability as well as oxidation resistance, highfluidity and good formability. With these properties, on one hand, theHEAs may be easily processed into different engineering components orused as structure materials. On the other hand, due to the low stackingfault energy of the FCC phase, the deformation twinning would beprevailed when the HEAs were deformed at low temperature, which in turnmaking the HEAs suitable for the application in the cryogenic field orlow temperature applications. The HEAs also possess high fluidity andcastability which make them possible for large-scale production. Inaddition, the method of the present invention involves easy andinexpensive procedures.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

Any reference to prior art contained herein is not to be taken as anadmission that the information is common general knowledge, unlessotherwise indicated.

1. A method of preparing a high entropy alloy structure comprising thesteps of: A. preparing an alloy by arc melting raw materials comprisingfive or more elements; B. drop casting the melted alloy into a cooledmold to form a bulk alloy; C. applying an external force against thebulk alloy to reshape the bulk alloy; and D. heat-treating the reshapedbulk alloy; wherein the bulk alloy is reshaped and/or heat-treated formanipulating the distribution of the microstructure therein.
 2. Themethod according to claim 1, wherein step C includes step C1 of rollingthe bulk alloy along a first direction to reduce the thickness of thebulk alloy.
 3. The method according to claim 2, wherein step C1 ofrolling is carried out along a longitudinal direction of the bulk alloy.4. The method according to claim 2, wherein the thickness of the rolledbulk alloy is reduced by 70%.
 5. The method according to claim 1,wherein formed bulk alloy includes a homogenous structure within whichthe microstructures are uniformly dispersed.
 6. The method according toclaim 1, wherein heat-treated bulk alloy includes a heterogeneousstructure within which the microstructures are non-uniformly dispersed.7. The method according to claim 1, wherein the crystals in themicrostructure are deformed during the heat treatment in step D to forma plurality of twins.
 8. The method according to claim 1, wherein step Dincludes step D1 of heating the bulk alloy to facilitate the movement ofthe microstructures.
 9. The method according to claim 8, wherein step Dincludes step D2, after step D1, of water quenching the heat-treatedalloy.
 10. The method according to claim 1, wherein each of the elementsis provided in an atomic percentage of 10% to 30%.
 11. The methodaccording to claim 1, wherein the elements are Cobalt, Nickel, Chromium,Iron and Aluminum.
 12. The method according to claim 11, wherein Cobalt,Nickel, Chromium, Iron and Aluminum are provided in an atomic ratio of30:30:20-0.5x:20-0.5x:x, with X being an integer of 14 to
 20. 13. Themethod according to claim 1, wherein the raw materials have a highpurity of >99.90%.
 14. The method according to claim 1, wherein step Aincludes step A1 of flipping and re-melting the raw materials in arepetitive manner.
 15. The method according to claim 1, wherein the moldis made of copper.
 16. The method according to claim 1, wherein thealloy is arc melted within a Ti-gettered argon atmosphere with apressure below 8×10⁻⁴ Pa.
 17. The method according to claim 8, whereinthe rolled bulk alloy is annealed at a temperature of at least 800° C.for 6 hours.
 18. A high entropy alloy structure prepared by the methodaccording to claim
 1. 19. The high entropy alloy structure according toclaim 18, wherein the alloy structure includes lamellar structures. 20.The high entropy alloy structure according to claim 19, wherein the sizeof the lamellar structures is provided in submicron range.
 21. The highentropy alloy structure according to claim 18, wherein the alloystructure possesses a hardness of 330 to 404 HV.
 22. The high entropyalloy structure according to claim 18, wherein the yield stress of thealloy structure is around 850 to 1000 MPa.
 23. The high entropy alloystructure according to claim 18, wherein the Young's modulus of thealloy structure is around 230 GPa.
 24. The high entropy alloy structureaccording to claim 18, wherein the alloy structure is thermal stable upto a predetermined temperature of 900° C.
 25. The high entropy alloystructure according to claim 18, wherein the structure is a dual phaseeutectic structure.
 26. The high entropy alloy structure according toclaim 25, wherein the dual phase includes ordered face center cubic(FCC) phase and body center cubic (BCC) phase.